With an increased emphasis on energy conservation today, the efficiency of lighting systems is receiving more attention. Gas discharge lamps, such as fluorescent lamps, can be quite efficient. These lamps work most efficiently when energized by high-frequency currents in the 25-100 kHz region. Circuits that produce such excitation are known in the art as "electronic ballasts", in order to distinguish them from conventional, inductive ballasts.
It is deemed desirable to operate fluorescent lamps at a preferred current level. This level is typically chosen to be the value that produces a fraction (usually 87.5%) of the light output of a standardized test environment. This test environment is specified in ANSI standards for the fluorescent lamp. When operated at the preferred current, the correct value of light output is obtained; long operating life of the lamp results; and the light output has the correct color index.
Ballasts provide several functions. Ballasts provide sufficiently high voltage to commence glow discharge within lamps. During the operation of lamps, ballasts provide a source impedance that overrides the negative resistance property of the glow discharge. A stable operating point results. For rapid-start and preheat lamps, ballasts also provide filament heater current. The most desirable ballasts provide these functions at low cost with high energy efficiency, reliability and safety, while minimizing distortion of AC line current and emission of radio frequency energy.
The user of a lighting fixture having multiple gas discharge lamps may desire to operate the fixture with fewer than the maximum possible number of lamps. This can be accomplished by installing fewer lamps in the lighting fixture than its maximum lamp capacity. Under these conditions, it is desirable to operate the installed lamps at their appropriate and correct current. As mentioned, maintaining lamp current at the correct value is an important consideration in preserving lamp life. For ballasts with lamps connected in parallel, however, removing one or more lamps from the fixture usually results in an increase in operating frequency, as well as a significant increase in lamp current. Heretofore, lamp current feedback has been used to correct for increased lamp current, but this corrective ability is not employed in most ballasts.
Lamps connected in parallel (i.e., a series network of a lamp and its ballast capacitor, connected in parallel with other such networks across the output of the ballast's power oscillator), represent a common type of connection for fixtures having three or four lamps; such an arrangement is even used with certain two-lamp fixtures. A problem with lamp current arises, however, because the ballast capacitors are coupled into the resonant circuit. Thus the ballast capacitors themselves partially determine the operating frequency. As lamps are removed or disconnected, the resonating capacitance decreases, which increases the operating frequency. However, the circuit voltage does not significantly change. With the increased operating frequency, the reactance of the ballast capacitors decreases. Since the ballast capacitors provide the main impedance to current flow, the lamp current also increases.
It would be advantageous to provide an electronic ballast to maintain appropriate and correct lamp current, regardless of the number of lamps connected to a given lighting fixture.
It would also be advantageous to provide a single electronic ballast for energizing and controlling a plurality of lamps.
It would be further advantageous to provide such an electronic ballast that uses a self-oscillating inverter, in which the lamps may be connected in a parallel.
It would be still further advantageous to provide such an electronic ballast that would incorporate a frequency-dependent control circuit.